docs: def label_values(self):
          """
          Return a list of all occuring label values.
  
          Returns:
              list: Lexicographically sorted list (str) of label values.
  
          Example:
              >>> ll = LabelList(labels=[
              >>>     Label('a', 3.2, 4.5),
              >>>     Label('b', 5.1, 8.9),
              >>>     Label('c', 7.2, 10.5),
              >>>     Label('d', 10.5, 14),
              >>>     Label('d', 15, 18)
              >>> ])
              >>> ll.label_values()
              ['a', 'b', 'c', 'd']
          """
  
          all_labels = set([l.value for l in self])
          return sorted(all_labels)